This application relates generally to gas turbine engine rotor blades and, more particularly, to methods and apparatus for reducing vibrations induced to rotor blades.
Gas turbine engine rotor blades typically include airfoils having leading and trailing edges, a pressure side, and a suction side. The pressure and suction sides connect at the airfoil leading and trailing edges, and span radially between the airfoil root and the tip. An inner flowpath is defined at least partially by the airfoil root, and an outer flowpath is defined at least partially by a stationary casing. For example, at least some known compressors include a plurality of rows of rotor blades that extend radially outwardly from a disk or spool.
Known compressor rotor blades are cantilevered adjacent the inner flowpath such that a root area of each blade is thicker than a tip area of the blades. More specifically, because the tip areas are thinner than the root areas, and because the tip areas are generally mechanically unrestrained, during operation wake pressure distributions may induce chordwise bending modes into the blade through the tip areas. In addition, vibrational energy may also be induced into the blades at a resonant frequency present during engine operation. Continued operation with such chordwise bending modes or vibrations may limit the useful life of the blades.
To facilitate reducing chordwise bending modes, and/or to reduce the effects of a resonant frequency present during engine operations, at least some known vanes are fabricated with thicker tip areas. However, increasing the blade thickness may adversely affect aerodynamic performance and/or induce additional radial loading into the rotor assembly. Accordingly, other known blades are fabricated with a shorter chordwise length in comparison to other known blades. However, reducing the chord length of the blade may also adversely affect aerodynamic performance of the blades.
In one aspect a method for fabricating a rotor blade for a gas turbine engine is provided. The method comprises forming an airfoil including a first side wall and a second side wall that each extend in radial span between an airfoil root and an airfoil tip, and wherein the first and second side walls are connected at a leading edge and at a trailing edge, and forming a rib that extends outwardly from at least one of the airfoil first side wall and the airfoil second side wall, extending outwardly from at least one of said first side wall and said second side wall, such that a natural frequency of chordwise vibration of the airfoil is increased to a frequency that is not excited by any excitation frequencies during normal engine operations.
In another aspect, an airfoil for a gas turbine engine is provided. The airfoil includes a leading edge, a trailing edge, a tip, a first side wall that extends in radial span between an airfoil root and the tip, wherein the first side wall defines a first side of said airfoil, and a second side wall connected to the first side wall at the leading edge and the trailing edge, wherein the second side wall extends in radial span between the airfoil root and the tip, such that the second side wall defines a second side of the airfoil. The airfoil also includes a rib extending outwardly from at least one of said first side wall and said second side wall, such that a natural frequency of chordwise vibration of the airfoil is increased to a frequency that is not excited by any excitation frequencies during normal engine operations.
In a further aspect, a gas turbine engine including a plurality of rotor blades is provided. Each rotor blade includes an airfoil having a leading edge, a trailing edge, a first side wall, a second side wall, and at least one rib. The airfoil first and second side walls are connected axially at the leading and trailing edges, and each side wall extends radially from a blade root to an airfoil tip. The rib extends extends outwardly from at least one of the airfoil first side wall and the airfoil second side wall, such that a such that a natural frequency of chordwise vibration of the airfoil is increased to a frequency that is not excited by any excitation frequencies during normal engine operations.